Look at the data sheet, there are some reserved characters which can be written to as though they are normal RAM but are actually in the font memory. If you send binary numbers to them the corresponding pixels in the LCD will activate. You need to draw the character you want, read it as rows of pixels and convert the 'on' pixels to '1' and 'off'' pixels to '0' to make binary numbers, you then write these numbers to the character RAM. After creating the new character you use it like any other.
Brian.